PISCES. 



67 



ORDER GANOIDEI. 



Tliis order derives its name from the character of the derma- 

 skeleton. Tliis consists usually of rhomboidal scales or plates, 

 sometimes detached spines, &c., but always consisting of a layer 

 of bone covered by a layer of peculiar enamel. Some Teleosts 

 have scales of the same character. 



The Ganoids have the vertebra-skeleton more or less cartilagin- 

 ous, the vertebrae being cartilaginous throughout life in the Pal- 

 aeozoic forms. The Bony Pike {Lepidosteus) has vertebrae con- 

 vexo-concave (opisthocoelus), a I'eptili'an feature, and the highest 

 vertebral structure of any fish. The ventral fins are always 

 placed far back and distant from the pectoral fins, and are often 

 lobed, with a surrounding fringe, whence Huxley's division, 

 Crossopterygidae. The tail is usually vertebrated, and was en- 

 tirely so befoi'e the Triassic. 



The oldest known fishes belong partly to this order, and they 

 are represented to-day by seven genera ; one {Accipenser) com- 

 mon to the jS^orthern Continents, four in North America, and 

 two in Africa (Newberry). 



The classification of the Ganoids is very diflacult. A simple 

 division is customary into Placo-ganoids, characteristic of Pal- 

 aeozoic time, and Lepido-ganoids, which range through all time 

 since the Devonian. The Placo-ganoids of the Devonian were 

 burdened with a defensive armor of immense thickness and 

 weight, but that method of defense has now been superseded by 

 swiftness and agility. 



No. 105. [309] Holoptychius nobilissimus, Agassiz. 



Body and Head, on 

 slab (cast). The Holop- 

 tychians were ganoid 

 fishes, belonging to 

 Huxley's group Cross- 

 opterygia ( ' ' fringe- 

 finned "), and have af- 

 finities with the Dipnoi, 

 being most closely rep- 

 resented now by Cera- 

 todus. Rhizodus, Dipterus and DinichtJiys belong in the same sub-order. This 

 genus is characterized by the large, deeply corrugated scales covering the 

 body, and the sculptured and granulated plates defending the head. The 

 teeth are of two kinds — small, serial teeth and large laniaries, the latter placed 



